Not applicable.
This invention is in the field of rapid prototyping, and is more specifically directed to the fabrication of three-dimensional objects by selective laser sintering.
The relatively new field of rapid prototyping has provided significant improvements in providing high strength, high density, parts useful for design verification and in pilot production. xe2x80x9cRapid prototypingxe2x80x9d generally refers to the manufacture of articles directly from computer-aided-design (CAD) data bases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has been reduced from several weeks to a matter of a few hours.
By way of background, an example of a rapid prototyping technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc. of Valencia, Calif., in which articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy at those portions of the powder layer that correspond to a cross-section of the article in that layer. Conventional selective laser sintering systems, such as the SINTERSTATION 2500plus system available from 3D Systems, Inc., position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The laser may be scanned across the powder in raster fashion, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that xe2x80x9cfillsxe2x80x9d the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, the next layer of powder is then dispensed, and the process is repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete.
More detailed descriptions of the selective laser sintering technology are provided by U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,132,143, and U.S. Pat. No. 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 assigned to 3D Systems, Inc., all incorporated herein by this reference. Laser power control systems for selective laser sintering systems are described in U.S. Pat. No. 6,085,122, issued Jul. 4, 2000, and in U.S. Pat. No. 6,151,345, issued Nov. 21, 2000, both assigned to 3D Systems, Inc., and also incorporated herein by reference. By way of further background, U.S. Pat. No. 5,352,405, issued Oct. 4, 1994 assigned to 3D Systems, Inc., and incorporated herein by reference, describes a method of scanning the laser across the powder in a selective laser sintering apparatus to provide a uniform time-to-return of the laser for adjacent scans of the same region of powder, thus providing uniform thermal conditions over the cross-section of each of multiple parts within the same build cylinder.
The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including polystyrene, NYLON, other plastics, and composite materials such as polymer coated metals and ceramics. Polystyrene parts may be used in the generation of tooling by way of the well-known xe2x80x9clost waxxe2x80x9d process. In addition, selective laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded in the fabricated molds; in this case, computer operations xe2x80x9cinvertxe2x80x9d the CAD database representation of the object to be formed, to directly form the negative molds from the powder.
FIG. 1 illustrates, by way of background, the construction and operation of a conventional selective laser sintering system 100. As shown in FIG. 1, selective laser sintering system 100 includes a chamber 102 (the front doors and top of which are not shown in FIG. 1, for purposes of clarity). Chamber 102 maintains the appropriate temperature and atmospheric composition (typically an inert atmosphere such as nitrogen) for the fabrication of the article.
The powder delivery system in system 100 includes feed piston 114, which is controlled by motor 116 to move upwardly and lift a volume of powder into chamber 102. Two feed pistons 114 may be provided on either side of part piston 106, for purposes of efficient and flexible powder delivery, as used in the SINTERSTATION 2500plus system available from 3D Systems, Inc. Part piston 106 is controlled by motor 108 to move downwardly below the floor of chamber 102 by a small amount, for example 0.125 mm, to define the thickness of each layer of powder to be processed. Roller 118 is a counter-rotating roller that translates powder from feed piston 114 to target surface 104. Target surface 104, for purposes of the description herein, refers to the top surface of heat-fusible powder disposed above part piston 106; the sintered and unsintered powder disposed on part piston 106 will be referred to herein as part bed 107. Another known powder delivery system feeds powder from above part piston 106, in front of a delivery apparatus such as a roller or scraper.
In conventional selective laser sintering system 100 of FIG. 1, a laser beam is generated by laser 110, and aimed at target surface 104 by way of scanning system 142, generally including galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled in combination with modulation of laser 110 itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. Scanning system 142 may scan the laser beam across the powder in a raster-scan fashion, or in vector fashion. Cross-sections of articles are often formed in a powder layer by scanning the laser beam in vector fashion along the outline of the cross-section in combination with a raster scan that xe2x80x9cfillsxe2x80x9d the area within the vector-drawn outline.
FIG. 2 illustrates a portion of target surface 104 at which four cross-sections 50 of one or more articles are being formed in a top layer of powder according to a conventional selective laser sintering method. In this example, cross-sections 50 are equally-sized rectangles, but at different angular orientations from one another relative to the x-y plane of target surface 104. As shown in FIG. 2, each of these cross-sections 50 are formed by raster scans of the laser beam across the powder of target surface 104, along scan lines 62. Also as shown in FIG. 2, each of the scan lines 62 are parallel to the x-axis in the coordinate system of target surface 104; as such, the x-axis is the xe2x80x9cfastxe2x80x9d scan axis for the raster scan of the laser beam, while the y-axis is the xe2x80x9cslowxe2x80x9d axis as it is the direction in which the raster scans advance upon completion of each scan.
According to the conventional technique illustrated in FIG. 2, the number of uniformly spaced raster scan lines 62 required to form a given cross-section 50 depends upon the orientation of the cross-section 50 in the x-y coordinate plane of target surface 104. In this example, four scan lines 62 are required to scan horizontally-oriented cross-section 50a. Eighteen and fifteen scan lines are required for angularly oriented cross-section 50c and 50b, respectively. Vertically oriented cross-section 50d requires thirteen scan lines 62. The spacing of scan lines 62 is selected by the operator of the selective laser sintering system, depending upon factors such as the desired structural strength of the resulting article, thickness of the powder layers, surface texture, and build speed. Thermal factors related to the scanning of the laser beam in the selective laser sintering process are described in the above-incorporated U.S. Pat. No. 5,352,405.
It has been observed, in connection with the present invention, that the number of raster scans (e.g., scan lines 62, in FIG. 2) is a significant factor in the overall time required to build an article by way of selective laser sintering. It has been discovered, in connection with the present invention, that a reduction in the number of scans performed in a given layer will therefore translate into a reduced build time, even if the scans are longer as a result. In addition, considering that the selective laser sintering apparatus must store vectors corresponding to the raster scan lines, in its memory, a reduction in the number of raster scan lines will result in fewer vectors to be stored in computer memory, and thus in more efficient use of computer resources.
It is therefore an object of the present invention to provide a selective laser sintering method and apparatus in which the build time of an article is reduced by reducing the number of raster scan lines required for each cross-section of the article.
It is a further object of the present invention to provide such a method and apparatus for which the computer memory requirements for storage of control vectors can be optimized.
It is a further object of the present invention to provide such a method and apparatus for fabricating an article with optimized tensile strength along its major axes.
It is a further object of the present invention to provide such a method and apparatus for fabricating an article for which rough surface area due to vector ends can be minimized.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
The present invention may be implemented into a method and apparatus for fabricating an article by selective laser sintering, in which the direction of raster scanning is optimized for each cross-section of the article. A computer associated with the selective laser sintering apparatus derives scan vectors for each article cross-section. In deriving the vectors, the scan time of the article cross-section is simulated or otherwise calculated, using several trial orientations of the cross-section. The scan vectors are then derived for the optimal orientation of the cross-section, and then stored in computer memory. The process is repeated separately for each cross-section to be formed in a given layer of powder, and for each layer of powder in the build cycle.